The present invention relates to a circuit capable of generating a reference voltage having a negative temperature coefficient, starting from a bandgap reference with a positive temperature coefficient.
In a large variety of electronic circuits, it is often required that certain control parameters maintain always the same preset values, independently of the variation of temperature to which the circuit may be subject. For example a parameter to be so controlled may be the maximum limiting current that can circulate through a load, that is, for example, through a power transistor driving an external load. Commonly, such a temperature stabilization is implemented by comparing the voltage drop on a sensing resistance through which the current to be controlled flows (which voltage drop signal is normally used for driving a control and regulation feedback loop) with a reference voltage.
For example, if the control and regulation loop must intervene always upon the reaching of the same output current value, it is necessary that the reference voltage vary with the temperature with the same law of the sensing resistance, in view of the fact that a resistor (in an integrated or discrete form) notably has a non-negligible temperature coefficient.
A circuit that is widely used for generating a voltage that varies according to a precise law with the temperature, is the so-called bandgap reference circuit, a functional diagram of which is depicted in FIG. 1. Such circuits are well-known to those of ordinary skill in the art of analog design. See, e.g., Feucht, D. HANDBOOK 0F ANALOG CIRCUIT DESIGN (1990); Gray and Meyer: ANALYSIS & DESIGN OF ANALOG ICS (2.ed.1984); Grebene, BIPOLAR & MOS ANALOG IC DESIGN (1984); all of which are hereby incorporated by reference.
A bandgap reference circuit as the one shown in FIG. 1, is based on the principle of exploiting variations of opposite sign with the temperature of two parameters, namely the base-emitter voltage Vbe (.apprxeq.-2 mV/.degree. C) and the so-called thermal voltage Vt (.apprxeq.+0.085 mV/.degree. C).
By referring to the diagram of FIG. 1, the bandgap voltage (Vbg) provided by the circuit is given by: EQU Vbg=Vbe 1+K.multidot.Vt (1)
wherein Vt is the "thermal voltage" kT/q, and K is a constant that depends on the values of the resistances RA and RB and the ratio n2/n1 between the emitter areas of the respective transistors Q1 and Q2.
Thus, by expanding the formula (1) one obtains: ##EQU1##
From the above formula (2) it may be observed that by varying the ratio RB/RA and/or n2/n1, a temperature coefficient of the Vbg that extends from -2 mV/.degree. C. through zero to positive values may be obtained.
An intrinsic limitation of this solution, consists in the fact that the variation of the bandgap voltage (Vbg) that is generated, does not remain linear for all possible values of T, but it may be considered linear only within a restricted range of variation of temperature that becomes wider with an increase of the coefficient K.
In other words, the equation (2) ceases to be valid beyond a certain temperature and the range of linearity that is associated with the bandgap circuit of FIG. 1, becomes relatively small if a negative temperature coefficient is desired for the produced bandgap voltage Vbg.
On the other hand, in many practical applications, it is required that the voltage variation remain linear for a relatively broad temperature range, for example -40.degree. C. to 150.degree. C.
A further drawback of the known bandgap reference circuits, is that the choice of the thermal coefficient and of the voltage Vbg that is generated are tied together in the sense that, once the value of one of these two parameters is fixed, the other is also automatically fixed.
Therefore, there is a need for a circuit capable of generating a reference voltage with a negative temperature coefficient, starting from a bandgap reference voltage having a positive temperature coefficient, in order to obtain a broad range of linear variation with a negative temperature coefficient.
This and other objectives and advantages are obtained by the circuit for generating a reference voltage with a negative temperature coefficient, object of the present invention.
Basically, the circuit of the invention allows generation of a reference voltage with a negative starting temperature coefficient, starting from a bandgap voltage having a positive temperature coefficient. Moreover, the selection of a certain temperature coefficient does not constrain the definition of the value of the reference voltage that is produced, thus allowing independent selection of temperature coefficient and reference voltage level.
In a sample class of embodiments, the circuit of the invention comprises a common, bandgap voltage generating network and an output amplifier, that, according to the invention, is provided with a feedback network which comprises a multiplier of a Vbe voltage.
In another sample class of embodiments, a Vbe multiplier circuit is functionally connected between an output node of the amplifier and a node of the bandgap voltage generating network onto which the bandgap voltage is generated, which is connected to ground through a resistance that fixes the current that circulates through the Vbe multiplier circuit. A resistive output voltage divider is functionally connected between the output node of the amplifier and ground.